The present invention relates to a gasoline direct injection engine using a piston having a recessed top surface.
Japanese patent laid open publication No. 2000-34925 discloses a gasoline direct injection engine using a piston having a recessed top surface. In a gasoline direct injection engine, fuel is injected directly into the combustion chamber as opposed to the more conventional fuel injection engine in which fuel is injected into an intake passage leading to the combustion chamber. In this previously proposed gasoline direct injection engine, a raised surface is provided around the recess formed in the top surface of the piston to adjust the compression ratio of the engine, and the pent roof of the combustion chamber and the raised surface are formed as mutually opposing parallel surfaces.
According to this previously proposed structure, as the raised surface of the piston and the roof surface of the combustion chamber come close together toward the end of the compression stroke, because the gap between the raised surface and the roof of the combustion chamber rapidly diminishes in size, the swirl flow of the intake air is forced directly into the recess. As a result, the mixture of a stoichiometric or otherwise prescribed air/fuel ratio formed around the spark plug is blown off, and the stability in the ignition of the mixture is impaired.
In view of such problems of the prior art, a primary object of the present invention is to provide a gasoline direct injection engine which ensures a stable ignition of the mixture at all times.
A second object of the present invention is to provide a gasoline direct injection engine which can operate under a lean bum condition in a stable manner.
A third object of the present invention is to provide a gasoline direct injection engine which can operate under a lean bum condition and is easy to manufacture.
According to the present invention, these and other objects can be accomplished by providing a gasoline direct injection engine, comprising: a cylinder block defining a cylinder; a piston slidably received in the cylinder and having a top surface defining a reference surface, the top surface including a recess recessed from the reference surface, a raised surface raised from the reference surface formed around the recess, and a sloping surface extending between the raised surface and the reference surface; a cylinder head attached to an end of the cylinder block to define a combustion chamber in the cylinder in cooperation with the piston, the combustion chamber defining a roof surface opposing the sloping surface; a fuel injection valve having a nozzle communicating with the combustion chamber; and a spark plug having an electrode facing the recess; wherein the sloping surface and the roof surface form mutually different angles with respect to a plane perpendicular to a cylinder axial line.
Thus, the intake air caught between the sloping surfaces of the piston and the opposing roof surface is gradually squeezed out of the gap between them, and is prevented from directly flowing into the recess at high speed. As a result, particularly under a low load condition involving a relatively small amount of fuel injection, the mixture in the recess is prevented from being blow away by a high speed swirl flow of the intake air flowing into the recess so that the air/fuel ratio near the spark plug electrode can be maintained at a proper level, and a stable ignition is ensured. In particular, this allows the engine to operate in a stratified charge lean bum mode over a wide operating range. According to a preferred embodiment of the present invention, the combustion chamber consists of a pent roof combustion chamber, and the sloping surface opposes an exhaust side of the roof surface, the sloping surface defining a shallower angle with the respect to the plane perpendicular to the cylinder axial line than the roof surface.
According to a particularly preferred embodiment of the present invention, a swirl shut off wall substantially surrounding the recess is provided between the recess and raised surface. This wall contributes to the prevention of the high speed swirl flow of the intake air directly flowing into the recess. In this case, a notch may be formed in the swirl shut off wall to avoid interference with the spark plug.
Preferably, a spray angle of the fuel injection valve is selected so that fuel injected by the fuel injection nozzle is charged substantially entirely into the recess at least in a stratified charge lean combustion mode. Thus, the injected fuel is substantially entirely charged into the recess so that a favorable air/fuel ratio is ensured for the mixture in the recess and wetting the cylinder surface with gasoline is avoided.